The only way to find out what will happen is to try to remove them. Penetrating oil applied a day or two in advance will help with potential success in removing the spark plugs. Also, you do not want to try to loosen them when then engine is hot unless you are a member of the extra-work-is-fun club. That is a surefire way to bring the threads out of the head with the spark plugs. If you do that, then you will have to replace the entire engine 
If the van has been driven the day you are to work on it, wait a couple hours before you start work. If you can comfortably rest your fingertips on the bare metal of the cylinder head, and it feels warm, not hot, to the touch, it is safe to work on. You may have better luck if the engine is this warm rather than stone cold, but either is okay. Hot engine is not okay.